In large bowl, beat together eggs, cheese, milk, onion, mustard, salt, if desired and pepper until well blended. Pour over reserved vegetables. Cover and cook over medium heat until eggs are almost set, about 8 to 10 minutes. Uncover and broil about 6 inches from heat until eggs are completely set in center, about 2 to 3 minutes OR let stand, covered, until eggs are completely set in center, about 8 to 10 minutes. Cut into wedges and serve from pan or either slide from pan or invert onto serving platter.